UPSTREAM: kasan: fix last shadow judgement in ...

... memory_is_poisoned_16()

The shadow which correspond 16 bytes memory may span 2 or 3 bytes.  If
the memory is aligned on 8, then the shadow takes only 2 bytes.  So we
check "shadow_first_bytes" is enough, and need not to call
"memory_is_poisoned_1(addr + 15);".  But the code "if
(likely(!last_byte))" is wrong judgement.

e.g.  addr=0, so last_byte = 15 & KASAN_SHADOW_MASK = 7, then the code
will continue to call "memory_is_poisoned_1(addr + 15);"

Signed-off-by: Xishi Qiu <qiuxishi@huawei.com>
Acked-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Andrey Konovalov <adech.fo@gmail.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Michal Marek <mmarek@suse.cz>
Cc: <zhongjiang@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
(cherry picked from commit 8d77a6d)
Signed-off-by: Alexander Potapenko <glider@google.com>

BUG=chromium:545468
TEST=KASAN reports on `modprobe test_kasan`

Change-Id: I90af4cfb6e3b4f37b4703e8a52667f37c326c236
Reviewed-on: https://chromium-review.googlesource.com/307976
Commit-Ready: Alexander Potapenko <glider@chromium.org>
Tested-by: Alexander Potapenko <glider@chromium.org>
Reviewed-by: Olof Johansson <olofj@chromium.org>

mm/kasan/kasan.c

@@ -135,12 +135,11 @@ static __always_inline bool memory_is_poisoned_16(unsigned long addr)
 
 	if (unlikely(*shadow_addr)) {
 		u16 shadow_first_bytes = *(u16 *)shadow_addr;
-		s8 last_byte = (addr + 15) & KASAN_SHADOW_MASK;
 
 		if (unlikely(shadow_first_bytes))
 			return true;
 
-		if (likely(!last_byte))
+		if (likely(IS_ALIGNED(addr, 8)))
 			return false;
 
 		return memory_is_poisoned_1(addr + 15);